1. Field of the Invention
The present invention relates to a process for forming a film having a photocatalytic function on the surface of a resinous base of, for example, a resinous article.
2. Description of the Related Art
Photocatalysts have photocatalytic activities for decomposing organic substances with the aid of light energy. When a film containing such a photocatalyst is formed on the surface of a resinous base and an organic substance is adhered to the surface of the film, the organic substance is decomposed by the photocatalyst and is thereby naturally removed.
However, if a film having a photocatalytic function (hereinafter referred to as xe2x80x9cphotocatalytic filmxe2x80x9d) is directly formed on the surface of a resinous base, the photocatalytic film decomposes not only the organic substance adhered to its surface but also an interface of the resinous base in contact with the photocatalytic film. As a result, the interface of the resinous base is deteriorated and the photocatalytic film may be peeled off from the resinous base.
As a possible solution to this problem, a process for the formation of a photocatalytic film has been proposed including the steps of forming an underlayer from a paint composition, such as a polymerizable and curable silicone composition, that is not decomposed by a photocatalyst on the surface of a resinous base, and forming a photocatalytic layer containing the photocatalyst on the underlayer. In such a two-layer photocatalytic film composed of an underlayer and a photocatalytic layer, the underlayer can prevent the decomposition of the resinous base by action of the photocatalyst in the photocatalytic layer. Thus it can prevent deterioration of the interface of the resinous base in contact with the photocatalytic film and prevent the photocatalytic film from peeling off from the resinous base.
Such photocatalytic films are generally used in surroundings which are exposed to light, and are likely to be subjected to abrasion or frictional force from external sources. Photocatalytic films therefore often require a high abrasion resistance. A possible solution to impart abrasion to photocatalytic films is the formation of an underlayer having a high hardness.
For example, polymerizable and curable silicone compositions include silane derivatives. Such silane derivatives can be classified as tetrafunctional, trifunctional, and bifunctional silane derivatives, as shown in FIGS. 2A to 2C, according to the number of functional groups which can be involved in hydrolysis reaction. Of these three types of silane derivatives, use of a tetrafunctional silane derivative can increase the number of bonds involved in polymerization and can increase the hardness of the resulting underlayer.
However, if large amounts of a tetrafunctional silane derivative are used, the hardness of the resinous base and the underlayer differ greatly from one another. As a result, volume changes in the layers, which may occur during the formation or use of the photocatalytic film, differ greatly from one another to thereby cause cracks in the underlayer. This problem is indicated in Japanese Patent Laid-Open No. 9-227829. This publication describes that a content of a hydrolyzable tetrafunctional silane derivative in the film exceeding 30% by mole may invite cracking in the film.
As is described above, if the hardness of the underlayer is merely increased, the resinous base and the underlayer will have largely different hardnesses, and cracking is liable to occur in the underlayer. If such cracks are formed, the adhesion between the resinous base and the underlayer is deteriorated to thereby cause peeling of the photocatalytic film from the resinous base.
Separately, a process for the formation of a photocatalytic film has been proposed which includes the steps of forming an underlayer as a primer resin layer on the surface of a resinous base, forming an intermediate layer composed of a thermally polymerizable and curable silicone composition on the underlayer, and forming a photocatalytic layer containing a photocatalyst on the intermediate layer, as is disclosed in Japanese Patent Laid-Open No. 10-67873. In such a three-layer photocatalytic film composed of an underlayer, an intermediate layer, and a photocatalytic layer, the underlayer has satisfactory adhesion to the resinous base and to the intermediate layer, and the photocatalytic film can be sufficiently prevented from peeling off from the resinous base.
However, a resulting photocatalytic film obtained by such a conventional process for the formation of a three-layer photocatalytic film composed of an underlayer, an intermediate layer, and a photocatalytic layer still has insufficient abrasion resistance.
Accordingly, it is an object of the present invention to provide a process which is capable of easily forming a photocatalytic film having satisfactory adhesion with a resinous base and having a higher abrasion resistance than conventional photocatalytic films.
Specifically, the invention provides a process for the formation of a film with a photocatalytic function. The process including: an underlayer forming step for forming an uncured underlayer from an organic composition on a surface of a resinous base, and polymerizing the organic composition to convert the uncured underlayer into an underlayer having a hardness higher than that of the resinous base; an intermediate layer forming step for forming an uncured intermediate layer from a polymerizable and curable silicone composition on the uncured underlayer or on the underlayer to yield an uncured intermediate layer, and polymerizing the polymerizable and curable silicone composition to convert the uncured intermediate layer to an intermediate layer, the polymerizable and curable silicone composition being prepared mainly from a hydrolyzable tetrafunctional silane derivative; and a photocatalytic layer forming step for forming a photocatalytic layer including a photocatalyst on the intermediate layer.
In the underlayer forming step, an organic underlayer having satisfactory affinity (compatibility etc.) with the resinous base is formed. The formed underlayer therefore has satisfactory adhesion with the resinous base.
In the intermediate layer forming step, an intermediate layer is formed from a polymerizable and curable silicone composition prepared mainly from a hydrolyzable tetrafunctional silane derivative. Hence, the formed intermediate layer may have a very high hardness. This intermediate layer has a higher hardness than intermediate layers of conventional photocatalytic films and therefore can provide a higher abrasion resistance of the resulting photocatalytic film than conventional.
The intermediate layer is formed on the underlayer having a higher hardness than the resinous base. Specifically, as difference in hardness between the underlayer and the intermediate layer is smaller than that between the resinous base and the intermediate layer, it is difficult for cracking, due to a difference in hardness with the underlayer, to occur in the intermediate layer. The intermediate layer therefore has a satisfactory adhesion to the underlayer.
In addition, the intermediate layer is formed from a polymerizable and curable silicone composition that is not decomposed by the photocatalyst, and which is resistant to decomposition by the photocatalytic layer formed in the subsequent photocatalytic layer forming step.
Thus, a described above, according to the present invention it is possible to easily form a photocatalytic film having satisfactory adhesion with a resinous base and having a higher abrasion resistance than conventional. These advantages can be particularly effectively obtained when the resinous base has a relatively low hardness.